This invention relates to a projection optical system having a magnification power varying device and more particularly to an optical system which illuminates a data record carrier, such as film or the like, and project the recorded information through lens systems of different magnifications, and which is particularly suitable for application to a micro-film reader/printer.
Generally, a projection optical system employed in a micro-film reader/printer is composed of a light source such as a lamp; a condenser lens which condenses light flux coming from the lamp and directs it toward the micro-film; and projecting lenses which project the light flux passing through the micro-film onto a screen or a photosensitive paper surface to form a magnified image thereon. In such an optical system, a so-called Koehler illumination method is employed to form an image of the lamp coming through the condenser lens systems approximately at a pupil point on the film end of the projecting lens. However, if the distance between the pupil point of the projecting lens and the film surface varies with each projecting lens employed, either the illumination area on the projection plane, e.g. the screen or the photosensitive surface, tends to become insufficient or the illumination on the projection plane tends to become uneven. To avoid this problem, the image of the lamp must be adjusted closer to the pupil point either by shifting the position of the lamp or by replacing or shifting a part or all of the condenser lens.
Let us assume that both a short focal length lens to be employed as projecting lens and a long focal length lens also to be employed as projecting lens are ordinary Gauss type lenses and that they are compared with respect to a distance g' between the pupil point on the film end of the projecting lens and the film surface. When the short focal length lens is of 13 mm and is used at 50 magnifications while the long focal length lens is of 29 mm and is used at 22 magnifications, the difference between them in the distance g' becomes about 19 mm. This necessitates adjustment of the condenser lens part as mentioned.
To solve this problem, methods have been proposed for maintaining the distance g' between the pupil point on the film surface end of each lens system and the film surface unvaried even where lens systems of varying focal lengths are employed. This appears in U.S. Pat. No. 3,588,226. In an example of such methods, the above stated distance g' is made longer by using an ordinary Gauss type lens as a long focal length lens and a retro-focus type lens as a short focal length lens. Hence the values of the distance g' of the two is made closer to each other. However, this method still requires extremely high manufacturing precision.
According to another example, the distance g' is shortened by using a rear diaphragm Gauss type lens as a long focal length lens while an ordinary Gauss type lens is used as a short focal length lens. However, the use of a rear diaphragm type lens ruins the symmetry of the lens group relative to the diaphragm. Then, as a result, it becomes difficult to correct comatic aberration and astigmatism. Furthermore, with such a method, the diameter of the lens facing the projection plane (a screen, a photosensitive surface, or the like) is inevitably increased to a considerable degree.